<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html {
            font-size: 20px;
        }

        p,
        span {
            /* font-size: 2em; */
            font-size: 2rem;
        }
    </style>
</head>

<body>
    <!-- 
        p: 2em -> 32px
        span: 2em -> 64

        1. 文字默认 16px 大小   16*2 = 32px
        2. 16*2 *2=64px
        3. 1em = 16px

        32/2 = 16

        p: 2rem -> 32px
        span: 2rem -> 32px
        32/2=16px
        root r 根 根元素
        修改 html 标签的 文字大小，就能修改到 rem 的基本大小
     -->
    <p> ppppp
        <span>sssss</span>
    </p>
</body>

</html>